[sdiy] best floating voltage-controlled resistor implementation?

[Tim Parkhurst]

On 4/29/06, Dino Leone <dino.leone at stanford.edu> wrote:
> I just read the discussion on the quadrature oscillator and realized that
> this is actually of great interest to me.
>
> I'm thinking of replacing the 100K pots (shape pots) in the timbre modulator
> (EN#72) we discussed some two weeks ago with voltage-controlled resistors.
> These are floating resistors in front of the zeners that chop the signal.
>
<snip>
>
> 3. (I might get garroted here for mentioning this): digital voltage
> controlled resistor. Disadvantage: limited to +/- 7V. and some digital
> circuitry (which could of course be abused to implement some other fancy
> stuff).
>
> Any ideas? The vactrol approach seems to be the easiest and most economical.
> But would it be fast enough?
>
> Best Wishes,
>
> Dino
>
Hi Dino,

First off, despite the fact that there are so many analog projects
discussed here, "digital" is not off topic or a 'bad word.' :-) In
regards to that, I'm not quite sure what you mean by a "digital
voltage-controlled resistor." There are digital 'pots' out there that
act much like a regular pot under digital control. The amount of
resistance usually is varied by incrementing or decrementing an
internal counter on the chip. Digital pots are available with anywhere
from 32 to 256 steps (or more if you really search). These would have
three disadvantages: 1) no direct analog control, 2) the 'stepping'
would be audible in many cases if you did control/modulate the pot in
real time and 3) as you mentioned, many of these digital pots are
meant for low-voltage applications.

As far as vactrols are concenrned, their speed (or lack thereof) can
be a problem. You can find them in varying speeds, and you can even
'roll your own' with a resistor and an LDR. Check the specs on the LDR
and get the quickest ones you can find. On the other hand, many people
find the distinct sound of the 'slower' LDRs and vactrols to be a
'feature' rather than a 'bug.' The only thing to do there is to
experiment with a few and see if you like the way LDRs react. There
are also a couple of demos at the Cyndustries site for their Low Pass
Gate that show the "Vactrol sound" off pretty well.
http://www.cyndustries.com/modules_quadgate.cfm


Tim (slow to react but several people like my sound) Servo
--
"Imagination is more important than knowledge." - Albert Einstein